Process for the preparation of citronellal

ABSTRACT

There is provided a novel process for selectively reducing citral to citronnellal by hydrogenation using wet palladium as catalyst in the presence of a base. It is a particular advantage of the process that citral from synthetic or natural sources may be employed to yield citronellal substantially free of byproducts.

United States Patent Easter, Jr. et al.

[4 1 Jan. 14, 1975 PROCESS FOR THE PREPARATION OF CITRONELLAL Inventors: William M. Easter, Jr., l-Iasbrouck Heights; Julian Dorsky, Rockaway; Robert F. Tavares, Cedar Grove, all of NJ.

Assignee: Givaudan Corporation, Clifton, NJ.

Filed: Mar. 25, 1970 Appl. No.: 22,658

US. Cl. 260/601 R, 260/566 A, 260/602,

260/631.5 Int. Cl. C07c 47/20 Field of Search 260/601, 601 R References Cited UNITED STATES PATENTS 7/1968 Forman et al. 260/690 FOREIGN PATENTS OR APPLICATIONS 1,086,447 10/1967 Great Britain 260/601 R OTHER PUBLICATIONS Ipatiew, Chem. Ber. Vol. 45, pages 3218-3224, 1912.

Primary ExaminerJoseph E. Evans Assistant Examiner-Roscoe Liles Attorney, Agent, or FirmThomas Cifelli, Jr.

[57] ABSTRACT 11 Claims, No Drawings PROCESS FOR THE PREPARATION OF CITRONELLAL DESCRIPTION OF THE PRIOR ART Citronellal is a very valuable ingredient of the perfumery and drug industry. It is convertible into hydroxycitronellal, isopulegol and menthol, which are valuable ingredients of perfume and flavor compositions, and in the drug industry and is found in Citronella Oil and other essential oils. The supply of citronellal from these natural sources is often subject to the vagaries common to agricultural products, such as poor crop years, political turmoil, hoarding, etc.

Prices fluctuate with these uncontrolled variables and industrial consumers frequently have great difficulty finding adequate quantities for their needs.

To avoid these problems, other sources of citronellal have been sought. Citral has always been considered to be an interesting raw material for citronellal. It is found in natural oils such as lemongrass oil and now is available at low cost from two synthetic routes, one starting with acetylene and the other with myrcene. However, it is difficult to convert citral to citronellal that is substantially free from undesirable by-products such as dimethyloctanal (DMA), citronellol, dimethyloctanol (DMO) and isopulegol. A very selective catalyst must be used so that of the two olefinic double bonds existing in the citral molecule in the 2 (allylic) and 6 positions, only the allylic double bond in the 2 position is reduced and the aldehyde group is not hydrogenated to the alcohols (citronellol and dimethyloctanol) and citronellal is not isomerized to isopulegol.

The following illustrate these reactions:

SUMMARY OF THE INVENTION Our invention provides an unexpected and greatly improved process for the hydrogenation of citral (neral & geranial) to citronellal having very little unreacted citral, a low content of DMA, and substantially no citronellol, isopulegol or DMO.

We have found that high yields of citronellal are obtained by hydrogenating citral with a palladium catalyst in the presence of small amounts of water and a base.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Palladium is conveniently used on a carrier or support such as carbon or alumina. Carriers containing from 1 to 10 percent palladium are commercially available and are satisfactory for this process. It is preferred to utilize 0.2 to 5 grams of 5 percent palladium on carbon or alumina per 100 grams of citral. To this is added the appropriate amount of water and base as discussed below.

Water must be present to achieve selective hydroge- Dimeth l octanal c o 4 i H 5 O k l i ll 1 H Geran1al 7 cltr one a m H .1

k! CHO H2OH-i- 10 Neral cit m e11o1 Dimethyloctanol The prior art reports several processes for hydrogenating citral to citronellal but none is economically attractive because of the low yields of citronellal. Skita (Ber. 42, l62736.) hydrogenated an alcoholic solution of citral using as catalyst an aqueous solution of PdCl containing gum arabic and obtained only a 33 percent yield of citronellal. Escourrou (Bull.Soc. Chem. 43, 1204-14.) used nickel catalyst but only obtained enough citronellal to identify by its semicarbazone. Bag and coworkers (Org. Chem. Ind. (U.S.S.R.) 2, 1414 (1936) CA. 31, 10O6 (1937).) reported a percent yield of citronellal from citral by hydrogenation with a Raney-type nickel catalyst prepared from an alloy containing 73 percent aluminum and 27 percent nickel. Slightly higher yields (64 69 percent) were re- Isopulegol nation and high yields of citronellal. The minimum amount required is about 0.3 gram of water per hundred grams of citral. Larger amounts such as equal parts of water and citral are also effectivev The use of about one to two grams of water per grams of citral is especially preferred. The water may be added to the mixture of citral, palladium and base. Water may also be added together with the palladium catalyst, as for example in the form of 5 percent palladium-on-carbon catalyst containing 50 percent water which is a convenient, free-flowing powder and is commercially available. When a small amount of water is added, say one gram per 100 grams citral, or when the water is added wet palladium-on-carbon catalyst it does not form a separate phase. Large amounts of water will, however,

not dissolve or be absorbed on the catalyst carrier and will form a two-phase liquid system which has not been found to be detrimental to the process.

Among the bases which may be employed in accordance with the present invention, the following may be mentioned:

1. inorganic bases including a. strong bases b. moderately strong bases and c. weak bases and very weak bases,

2. organic bases, such as amines, preferably tertiary amines including a. acyclic tertiary amines and homocyclic tertiary amines b. heterocyclic tertiary amines (ie: those having the amine nitrogen as part of a cyclic system, including unsaturated and saturated heterocyclic amines.) There may be utilized inorganic bases as follows:

la. The strong bases (pK l These include the hydroxides of alkali metals, for example lithium, sodium and potassium hydroxides. Also included in this group are compounds which are converted into these hydroxides on contact with water. Among.

this group may be named alkali metal hydrides, alkali metal acryls and alkyls, and alkali metal alcoholates, for example sodium hydride, lithium hydride, butyl lithium, phenyl lithium, potassium methoxide and the like.

1b. A second group of bases are the moderately strong bases varying from about pK 1.0 to pK 4. These include alkaline earth metal hydroxides such as calcium hydroxide and non-hydrated oxides such as silver and lead oxides.

lc. The third group of bases are the weak (pK,, 4 5) and very weak bases (pK 5). These include hydroxides such as ammonium hydroxide and the salts of strong bases with weak or very weak acids. For the purposes of this definition a strong base has a pK l, a weak acid has pK 1. It is understood that only those salts which have a pK,, 4 are included and that some bases included in the first and second group can be produced by reacting a sufficiently strong base with a sufficiently weak acid.

The strong bases are those defined more specifically- The second group of bases which may be utilized are organic bases. All organic bases may be used, that is to say compounds having a CN bond wherein the nitrogen atom effectively has a free electron pair. These bases may be broadly classified as primary, secondary and tertiary amines and include saturated and unsaturated acyclic homocyclic and heterocyclic amines. While amides could be used, the pl(,, of most amides is too high to achieve satisfactory results. It has been found that while primary and secondary amines may be used, tertiary amines are especially preferred.

Primary and secondary amines could react with the aldehyde. The quantities used are of such an order that the yield will not be appreciably reduced. It has been found that the use of primary and secondary amines gives a product which is superior in yield and purity to that obtained from use of a wet catalyst without base. However the yield and purity is not as good as that obtained from tertiary amines. The tertiary amine of choice is triethyl amine since it is cheap and a stable article of commerce. Other tertiary amines which may be used include acyclic and homocyclic tertiary amines as well as heterocyclic amines the last being those wherein the nitrogen atom forms part of the cyclic moiety.

The organic bases include:

2a. Acyclic and homocyclic tertiary amines, among which maybe mentioned: trialkyl amines, trialkarylamines, triaryl amines and amines having mixed substituents; for example, trimethyl amine, tripropylamine, tributylamine, tribenzylamine, tritolylamine, triphenylamine, trinaphthylamine, dimethylphenylamine, diphenylethylaminc, dibenzylmethylamine and the like.

The organic bases also include:

2b. The cyclic tertiary amines namely the unsaturated heterocyclic amines. For example, there may be included: N-alkyl pyrroles such as N-ethyl pyrrole, N aryl pyrazoles such as N phenyl pyrazole, pyridine, quinoline, and the nuclearly substituted derivatives thereof. Also included are the unsaturated heterocyclic tertiary amines such as N alkyl pyrrolidine, N,N-diaryl piperazines such as N,N' -dinaphthyl piperazine and N alkyl morpholines such as N ethyl morpholine.

As stated above, there may also be used primary and secondary amines, although these are not preferred.

Thus there may be used acyclic and homocyclic primary amines such as alkylamines, aryl amines, aralkylamines and alkyl amines for example methylamine, ethylamine, aniline, benzylamine, toluidine and the like.

There may also be used the corresponding groups of N,N-disubstituted (i.e. secondary) amines for example dialkylamines, diarylamines, diaralylamines and dialkarylamines as well as amines substituted by a combination of said substituent groups such as diethylamine, dipropylamine, diphenylamine, dinaphthylamine, dibenzylamine and dixylylamine as well as methyl phenylamine, phenylnaphthylamine, propylbenzylamine and the like.

As secondary amines there may also be included the non substituted analogs of the saturated cyclic amines mentioned above as well as the unsubstituted analogs of those unsaturated cyclic amines mentioned above as being N substituted. For example: pyrrolidine, piperidine, piperazine, morpholine, pyrrole and pyrazole may be used.

The quantity of base used varies inversely as the strength of the base. The range of based used lies between 0.1 and 10 grams of base per grams of citral. Thus when a strong base, such as sodium hydroxide is used between about 0.1 and about 0.5 grams are preferred. A moderately strong base such as sodium carbonate calls for from about 1 to about 3 grams of base, and a weak base such as triethylamine requires from about 4 to about 8 grams of base. Although primary and secondary amines are not the amines of choice, when used, from 1.5 to 5, but preferably not exceeding 5 grams of base should be employed. All of the above quantities being relative to 100 grams of citral charged.

The above listing of organic amines is not intended to be limiting. It is set forth for purposes of illustration only, it being understood that one skilled in the art could readily find other bases which could fall within out further purification for certain manufacturing purposes such as the preparation of hydroxycitroncllal. If a purified form of citronellal is desired it can be obtained by distilling the hydrogenation product.

cess. Excellent results were obtained with pure citral (96 percent) and with crude citrals (70 to 75 percent) from both natural and synthetic sources. Thus, it is possible to start with crude citral and avoid the cost or purifying the starting material.

Citronellal prepared by our process can be used withthe general classification and be operative. 5

Using the above-mentioned catalyst and conditions, EXAMPLE 1 the hydrogenation is rapid and specific. Hydrogen ab- Pure citral, (76grams), 2 grams of wet 5 percent palsorption stops when about the stoichiometric amount ladium on carbon (50% H and 1 gram of sodium of hydrogen has been added. The hydrogenation may carbonate was hydrogenated at 27-30C and 40-50 be conducted at from about to about 150C at prespsi of hydrogen. After 5 hours the hydrogen absorption sures of from about 5 to about 600 psi. We prefer to opstopped. The oil was filtered and the reaction product erate the process at about 25-40C and about 40-50 analyzed as follows: psi. The reaction product contains more than 96 per- Purity by oximation: 94.3 percent cent citronellal, less than 4 percent dimethyloctanal, a VPC analysis: 3.7 percent dimethyloctanal, 96.2 pertrace of citral, and no isopulegol or citronellol. 5 cent cit o ll l,

The advantages of the process and invention are 0,1 percent it l, no i ll l or clearly shown by comparison with an experiment using i l l nickel as Catalyse In this ease the hydrogenation The pure citral utilized was obtained from lemongrass rate did not slow down when the stoichiometric amount il d h d h f ll i compositions; of hydrogen for converting citral to citronellal was ab- 1 P i 95 5 percent b oximation sorbed and hydrogenation continued if allowed. The 2 B li i chromatography (VPC) composition of the oil when the hydrogenation was Geranial 33; 7 pp after the stoichiometric amount of hydrogen In accordance with the above procedure, but where had been absorbed was 2 Pereent dinlethyleetanal, 55 in place of sodium carbonate there is utilized dilute percent citronellal, percent citronellol and 18 per- 5 aqueous sodium hydroxide the same product is obcent citral. tained in substantially the same degree of purity.

The advantages of using a combination of water and a base with the palladium are outstanding. Dry palla- EXAMPLE H dium on carbon is a poor catalyst for the hydrogenation Pure citral (76 Grams), from the same source as utiof citral to citronellal. The catalyst is slowly deacti- 3O lized in Example I, 1 gram of dry 5 percent palladium vated by high concentrations of citral, and to effect abon r 1 gr m f water an 1 gra of i m rsorption of one mole of hydrogen it is necessary to bonate was hydrogenated at 4550C and -50 psi of make two or three additions of dry palladium on carhydrogen. After 3.5 hours the hydrogen absorption bon catalyst. As the hydrogenation progresses and the stopped. The reaction product had the following analy concentration of the citral drops below about 10 per- 5 sis: cent the catalyst remains active but loses its selectivity P rity by O imation 4.5 percent and a considerable amount (22 percent) of dimethyloc- VPC analysis: 3.4 percent DMA, 96.5 percent citrotanal is formed. Palladium on carbon plus water with-' nellal out the base or palladium on carbon plus base without 0.1 percent citral, no citronellol or the water are also poor catalysts. The oil from the for- 40 isopulegol. mer system when the hydrogen absorption stops con- In accordance with the above procedure but where, tains 2.1 percent DMA, 33,7 percent itronellal, 5,3 in place of sodium carbonate there is utilized calcium percent isopulegol and 58.9 percent citral. The latter I hydroxide, the same product is obtained in substanwhen the hydrogen absorption stops contains 8.0 pertially the same degree of purity. cent DMA, 80 percent citronellal and 12 percent citral.

It is an additional advantage of the present process EXAMPLE m that it can be used with citral from both natural and Pure it al (from the Same Source as synthetic sources. Excellent results have been obtained lized in example 2 grams of Wet 5 Percent Palladium with natural citral from lemongrass oil and with synon carbon (5 2 n 1 g of Sodium CHThOIwtQ thetic citralprepared by rearrangement ofdehydrolina was hydrogenated at various temperatures and preslyl acetate (derived from acetylene) and synthetic citsures. The conditions and results are shown in Table I.

TABLE I Purity V.P.C. Temp" Time by oxi- Citron- Citrom lso- C PSIG hrs mation DMA ellal Cilral ellol pulegnl ral prepared by oxidation of geraniol (derived from EXAMPLE IV fl'p Via yrcene). Pure citral (100 Grams), from the same source as uti- The purity of the citral used is not critical in the pro- Zed in Example 1 3 grams f dry 5 percent mdium on alumina. 1.3 grams of water and 1.3 grams of sodium carbonate, was hydrogenated at 3540C and 40-50 psi of hydrogen. After 7 /2 hours the hydrogen absorption stopped. The reaction product had the following analysis:

Purity by oximation 93.1 percent VPC analysis: 1.8 percent DMA, 96.8 percent citro- In accordance with the above procedure but where, nellal, in place of sodium carbonate there is utilized pyridine, 1.4 percent citral, no citronellol or quinoline, N methylpyrrole, or N methylpiperidine, isopulegol. there is obtained the same product in substantially the In accordance with the above procedure but where same degree of purity. in lace of sodium carbonate there is utilized ammoniu in hydroxide; potassium acetate or sodium benzo- EXAMPLE ate, the same product is obtained in substantially the Ci l (33 grams), 2 grams of wet 5 percent lmsame degree of purity. dium on carbon (50% H 0) and 1 gram of sodium carbonate was hydrogenated at 45C. and 40-50 psi of hy- EXAMPLE V drogen. After 6 hours the hydrogen absorption Crude Citral, (2,000 g 40 grams of W61 5 P stopped. The oil produced was filtered and the reaction cent palladium on carbon (50% H 0) and 40 grams of mixture had the f ll i analysis; Sodium Carbonate was hydrogenated at and 45 P V.P.C. analysis: 3.4 percent DMA 96.6 percent Citof hydrogen. After 5 hours the hydrogen absorption ronellal stopped. The oil was filtered and the reaction product 0.6 percent Citral by UV had the following analysis: Purity of oximation: 88.5 percent Purity by oximation 81.4 percent Citral utilized was obtained by vacuum distillation of V.P.C. analysis: 5.4 percent DMA, 93.6 percent citlemongrass oil nd h d the following analysis:

ronellal l. 91 percent by oximation 1.0 percent, no citronellol or isopulegol 2. V.P.C. ratio The crude synthetic citral utilized was prepared from Neral: geranial 33.67 dehydrolinalyl acetate and had the following analysis:

1. 71.9 percent citral by U.V. EXAMPLE 1 Percent Purity by oximation Pure citral, (100 grams) (from the same source as y VPC Neral: Geranial 38362 utilized in example 1) was hydrogenated at C and In accordance with the above Procedure but Where -50 psi of hydrogen until the hydrogen absorption in Place of Sodium Carbonate there is utilized trlethyl' stopped. The conditions and results are shown in Table amine, or dimethylphenylamine, there is obtained the 11 TABLE 11 Sod i- V.P.C.

um Car Catalyst bon- Purity grms grms ate Time Citronlso by oxiwct* dry** grms hrs DMA ellal pulegol Cirtal mation We! 57: Palladium on carbon H O) "Dry 5'7: Palladium on carbon same product in substantially the same degree of purity.

EXAMPLE VI 8 Purified citronellal can be recovered from the hydrogenation product by distillation. Using a 1 inch diameter by 9 inch long Goodloe column there was obtained 1,300 grams of 92 percent pure citronellal.

Thecrude, synthetic citral utilized was obtained from myrcene and had the following analysis:

1. 76 percent Citral by U.V.

2. 77.4 percent Purity by oximation 3. By VPC:

Neral: Geranial 30:70

A study of Table ll clearly shows the advantage of our process. Dry 5 percent palladium on carbon alone or with sodium carbonate are poor catalysts: wet 5 percent palladium on carbon (50% H O) without the sodium carbonate is a poor catalyst.

What is needed is a combination of palladium on carbon, water and base for the successful hydrogenation of citral to citronellal.

EXAMPLE lX Pure citral, grams), (from the same source as utilized in example 1) was hydrogenated at 35-40C and 40-50 psi with 1 gram of 5 percent palladium on dry carbon and 1 gram of sodium carbonate using varying amounts of water until the hydrogen adsorption stopped. The results are shown in Table 111.

TABLE 111 V.P.C.

Grms Time Citrolso- Purity by H O Hrs DMA nellal pulegol Citral oximation Table Ill clearly shows the advantage of using water in combination with 5 percent palladium on dry carbon and base. A two phase system resulted when 5 grams of water was used.

4. A process according to claim 1 wherein the palladium is in the form of palladium deposited on a carrier.

5. A process according to claim 4 wherein the carrier is carbon or alumina.

EXAMPLE x 5 Process ccordin to claim 5 wherein the catalyst consists essentially of R f (100 grams) (from the same some? as a. l-l percent palladium on carbon or alumina utilized in example I) was hydrogenated at 40-50 psi of water hydrogen g Y j 39$ i hlydrogenhabsorP- c. a base selected from the group of bases consisting tTiorgls pspe 6 "10115 an t are 5 Own m of alkali metal hydroxides, alkali metal carbonates a e and tertiary amines.

TABLE IV V.P.C.

Puriy y Catalyst Oxi- Citgrms grms Base Temp Time ma ron lso- Citwet" dry** grms C Hrs tion DMA ellal pulegul ral 2 Sodium 4 94.5 3.8 95.5 0.0 07

C31" bonate 1, 2 Tri- 38 9 4.4 94.9 0.0 0.6

ethyl amine 5. 2 Sodium 3 93.6 4.4 95.1 0.0 0.5

30 Hydroxide l, of 10% aqueous "Wet 5% palladium on carbon (50% HQO) "Dry 5% palladium on carbon EXAMPLE XI 7. A process according to claim 6 wherein the cata- Pure citral, (152 grams), (from the same source is lystt used Conslsts fgssennauli of utilized in example I) and 5 grams of Raney nickel type betwefm by of 5 percent catalyst were hydrogenated at 70C and 50 psi hydro- Pal lad'um 9 Carbon or alumma P 100 Parts gen pressure. In 6% hrs. about the stoichiometric Weght of citral charged. amount of hydrogen for hydrogenation of citral to citat l ea5t Pdrts by Weight of Water P 100 Parts ronellal had been added. The reaction product, by of Cmal Charged VPC, showed the following analysis: 0. between 0.1 and 10 parts by weight of base per 100 DMA 2 percent, Citronellal 55 percent, Citral 18 Parts y Welght f cltral h percem and 8. A process according to claim 6 wherein the hydro- (ji ll l 25 percent. genation is carried out at between about 25 to about The above example clearly shows the poor selectivity 40C at from about 40 to about psi. of the Raney nickel type catalyst for the hydrogenation 9. A process according to claim 8 wherein there are of citral. used What is claimed 50 b. between about I to about 2 parts by weight of l. A process for the preparation of citronellal which water comprises hydrogenating citral in the presence ofa catbetween about to about 3 Parts by welght of alyst consisting essentially of palladium, water and a l or Wtasslum carbonate P 100 Parts by base at between about 5C and about 150C and a presweght of Charggdsure of between about 5 psi a d about 6 P the mini- 10. A process according to. claim 8 wherein there are mum amount of water being about 0.3 grams of water d per hundred grams of citral and the amount of base b, b tween about I to about 2 parts by weight of being between 0.1 and 10 grams of base per grams water of Citfal- 0 c. between about 3-6 parts by weight of triethyl 2. A process according to claim 1 wherein the base is selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, salts of strong bases with weak acids and amines.

3. A process according to claim 2 wherein the salts 5 of strong bases with weak acids are alkali and alkaline earth metal carbonates, bicarbonates, acetates and benzoates, and the amines are primary, secondary and tertiary amines.

amine per 100 parts by weight of citral charged.

ral charged. 

2. A process according to claim 1 wherein the base is selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, salts of strong bases with weak acids and amines.
 3. A process according to claim 2 wherein the salts of strong bases with weak acids are alkali and alkaline earth meTal carbonates, bicarbonates, acetates and benzoates, and the amines are primary, secondary and tertiary amines.
 4. A process according to claim 1 wherein the palladium is in the form of palladium deposited on a carrier.
 5. A process according to claim 4 wherein the carrier is carbon or alumina.
 6. A process according to claim 5 wherein the catalyst consists essentially of a. 1-10 percent palladium on carbon or alumina b. water c. a base selected from the group of bases consisting of alkali metal hydroxides, alkali metal carbonates and tertiary amines.
 7. A process according to claim 6 wherein the catalyst used consists essentially of a. between 0.2 and 5.0 part by weight of 5 percent palladium on carbon or alumina per 100 parts weight of citral charged b. at least 0.3 parts by weight of water per 100 parts of citral charged c. between 0.1 and 10 parts by weight of base per 100 parts by weight of citral charged.
 8. A process according to claim 6 wherein the hydrogenation is carried out at between about 25* to about 40*C at from about 40 to about 50 psi.
 9. A process according to claim 8 wherein there are used b. between about 1 to about 2 parts by weight of water c. between about 1 to about 3 parts by weight of sodium or potassium carbonate per 100 parts by weight of citral charged.
 10. A process according to claim 8 wherein there are used b. between about 1 to about 2 parts by weight of water c. between about 3-6 parts by weight of triethyl amine per 100 parts by weight of citral charged.
 11. A process according to claim 8 wherein there are used a. between about 1 to about 2 parts by weight of water b. between 0.05 and 0.5 parts by weight of sodium or potassium hydroxide per 100 parts by weight of citral charged. 